This application claims priority from Australian Provisional Patent Application No. 2005901353, the entire content of which is hereby incorporated by reference.
The most commonly deployed mobile location systems at present are cell ID and enhanced cell ID systems. These systems use existing measurements within the mobile network. As a result they are able to locate existing handsets without requiring the modifications typically required by higher accuracy techniques.
Cell ID only systems typically use the association between a mobile and its serving cell and return a corresponding position fix. In some cases this position may simply be the coordinates of the base station. In the case of sectored sites, a better solution is usually the notional centroid of the coverage area of that cell.
Enhanced cell ID systems add further information to the serving cell identity to achieve better performance. The most common piece of information is a round trip delay measurement between the serving cell and the mobile. Using the GSM network as an example, the Timing Advance (TA) is measured by the base station and represents the round trip delay from the base station to the mobile and back again. It is encoded with a resolution of one bit and therefore provides a one way range measurement with a resolution of half a bit or approximately 550 meters. The use of a TA based range constraint enables a significant accuracy improvement over a CID only solution. The actual degree of improvement depends on several factors but is typically a factor of two to three times.
A further source of information used by some enhanced cell ID systems is signal levels measured either by the handset or the network. For instance in some networks, each cell broadcasts a beacon channel to aid handsets in selecting the optimal point through which to access the network. Handsets measure the beacon signal power received from the neighbouring cells. If the transmit power of these beacon channels is known then the power received at the mobile reflects, to some degree, the range between base station and mobile. Other factors such as antenna radiation pattern and fading in the radio channel also affect the power received. In a system already using serving cell identity and round trip time, the additional use of signal level measurements provides a moderate level of improvement. Typical improvement factors range between 1.2 and 2.0 times.
It is an object of the present invention to provide a further or alternative method and system for improving the location of a mobile radio terminal within a radio communications network.